Various international tax related resources.

European Union Resources
EU National Tax Websites
European Commission - Taxation & Customs Union
Comprehensive resource covering tax and tariff information within the European Customs Union.
European Commission VIES VAT number validation
You can verify the validity of a VAT number issued by any Member State by selecting that Member State from the drop-down menu provided, and entering the number to be validated.
Search for VAT numbers over 30 million companies from 26 European countries.

International Bureau of Fiscal Documentation
IBFD claims that it is the world’s foremost authority on cross-border taxation.
Intra-European Organisation of Tax Administrations
A non-profit intergovernmental organisation, which provides forum to assist its members to improve tax administration. The main field of its activity is organising workshops which provide opportunity to participants from various member tax administrations to exchange views, experiences and best practices about the most important and current issues in the field of taxation. IOTA also promotes cooperation between tax administrations in the European region and supports their development according to their individual needs.

Tax Related Services
International Sales Tax Refund
International Sales Tax Refund Corporation is operated by European VAT (Value Added Tax) experts.
Developer of transaction based, worldwide commercial tax compliance software.